In recent years, digital modulation techniques of high quality and high frequency utilizing efficiency have been developed for a transmission of video or audio signals. Particularly in the mobile radio communication, an adoption of orthogonal frequency division multiplexing (hereinafter referred to as OFDM) modulation technique which is durable against a multiple path interference is now under consideration. OFDM is a system that disperses transmission digital data into multiple carriers (approximately 256 through 1024 carriers) (hereinafter simply referred to as multiple carriers) which are mutually orthogonal and that modulates each of them.
FIG. 1 is a waveform diagram showing a typical frequency spectrum of the OFDM modulated wave.
As shown in FIG. 1, an OFDM modulated wave is comprised of multiple carriers, each of the carriers having been, for instance, processed using QAM (quadrature amplitude modulation). In FIG. 1, item 1 represents a sub-carrier power spectrum, item 2 represents an OFDM power spectrum, and item 3 represents channel bandwidth. The vertical axis represents power. Frequency power spectrum of the OFDM modulated wave in a channel is expressed by superposing a frequency spectrum of multiple QAM modulated carriers. Further, because all the frequency spectra of the carriers have the same characteristics and because the carriers are provided at a relatively small and/or the same frequency interval, the waveform of the OFDM modulated wave resembles white noise.
The OFDM modulated wave is transmitted after it is orthogonally modulated. The OFDM modulated wave is obtained at a receiver section through synchronous demodulation carriers for sync demodulation are detected by controlling the oscillation output of an oscillator using a transmitted wave. However, since the transmitted OFDM modulated wave is a waveform resembling white noise, it is not possible to eliminate frequency detuning using the transmitted OFDM modulated wave. So, in a conventional OFDM sync demodulation circuit, a frequency deviation is reduced by improving the accuracy of an oscillator. FIG. 2 is a block diagram showing a conventional OFDM modulation/demodulation system including an OFDM sync demodulation circuit, which is discussed in "1992, Collection of Release Scheduled Open Research Theses of NHK Technical Research Institute", pp. 28-36.
In FIG. 2, an OFDM modulation circuit 1 orthogonally modulates the OFDM modulated transmitted data using a carrier of frequency f1, and outputs the data from an adder 2 after OFDM modulating transmission data by an inverse fast Fourier transform circuit (hereinafter referred to as IFFT circuit). The OFDM modulated wave is input to an OFDM sync demodulator 3 via an adder 4. The OFDM modulated wave is supplied to a BPF 5, and after removing noise, it is supplied to multipliers 6 and 7. The multiplier 6 applies an in-phase axis according to a carrier having frequency f1 from an oscillator 8; multiplier 6 performs an in-phase detection through multiplication with the OFDM modulated wave. Further, the phase of the carrier output from the oscillator 8 is shifted by -90.degree. using phase shifter 9 before being input to the multiplier 7 which performs orthogonal detection through multiplication with the OFDM modulated wave.
The detection outputs from the multipliers 6 and 7 are respectively applied to A/D converters 12 and 13 via low-pass filters (LPF) 10, 11, respectively and are converted to digital signals. The outputs of the A/D converters 12 and 13 are applied to a fast Fourier transform (hereinafter referred to as FFT) circuit 14, where carriers are demodulated. The demodulated signal output from the FFT circuit is converted into serial data and output by a parallel/serial converter (hereinafter referred to as P/S converter) 15.
In a system, as shown in FIG. 2, frequency deviations are reduced by improving oscillation accuracy of oscillator 8 as described above. However, it is extremely difficult to maintain high oscillation accuracy. Furthermore, a high accuracy oscillator is expensive and hard to incorporate into commercial models of receivers.
Further, because the OFDM modulated wave resembles white noise, it is also difficult to maintain a highly accurate detected clock frequency using an OFDM modulated wave. Therefore, conventional system using OFDM processing inserts a reference signal to obtain clock synchronization. For instance, a reference signal such as a non-signal period (null symbol period) or a slot, etc., is added to data for every several tens of symbol periods. As such, conventional systems achieve a clock synchronization by detecting a reference signal contained in transmitted data. That is, by detecting a demarcation timing of a reference signal from the envelope of the modulated wave, the clock synchronization is obtained on the basis of the detected timing.
However, a sufficient accuracy cannot be obtained by the method of obtaining the clock synchronization based on a reference signal which is periodically transmitted. Further, the reference signal may be disturbed and/or detected erroneously so that the normal demodulation can not be executed for an extended period of time, e.g., until a next reference signal is detected.
In conventional OFDM sync demodulation circuits as described above, a highly accurate oscillator was needed to obtain carrier synchronization. Thus, in the conventional circuits, it is difficult to adopt such a highly accurate oscillator for commercial models of receivers. Further, conventional systems experienced problems when implementing the above-described method of detecting clocks by inserting a reference signal in transmission signal. Specifically, the carrier synchronization is low in accuracy and weak against disturbance.